HIV infection in vitro and in vivo was demonstrated by in situ hybridization using HIV RNA or DNA specific radiolabelled or biotinylated probes. The cell types infected by HIV were similar to those found to express viral antigens but important quantitative differences existed. Antigen expression may have represented phagocytosis of virus in several cases. Human brain lymphomas were found to have focal expression of HIV antigens in tumor cells and macrophages. The implications of these findings are not known. Rabbit and simian models of lentiviral infections and disease were further developed and assessed (cf. project Z01CPO5092). Important differences were found between these infections and those in humans. Rabbits had low grade persistent infections, while macaques expressed abundant antigen only in selected tissues and patas monkeys appeared not to express SIV-related antigens naturally. Three specific common tumors of rodents were studied for histogenesis, etiology and pathology. Histiocytic sarcomas in mice were found to express macrophage-specific genes including c-fms and were induced to express CSF1, TNF-Alpha, IL-1gamma, and IL-1beta. MAC-2 and lysozyme were found in autopsy specimens. These findings provide, for the first time, important information on the origin of these common mouse tumors. A cell line expressing well-differentiated macrophage antigens and genes was developed and characterized and may be the only available such line not infected with viruses. Hamster lung tumors, induced by N-nitrosodiethylamine, were found for the first time to originate from cells migrating from bronchiolar epithelium using antigenic markers. A new rodent model for human biphasic mammary gland adenomyoepitheliomas and carcinomas was developed with DMBA in B6D2FI mice.